In short: A kilowatt (kW) measures power, how fast electricity is being used or produced at a moment in time. A kilowatt-hour (kWh) measures energy, the total amount used or produced over time. Your electricity bill is charged in kWh, and solar panels are sized in kW. To size a solar system, you convert your annual kWh usage into the kW of panels needed to produce it.
If you’ve ever been confused by solar quotes that mix kW and kWh, you’re not alone. The two are easy to mix up, and mixing them up leads to bad decisions. This guide explains the difference in plain language, shows you where to find your numbers, and walks through an example of sizing a system from your energy bill.
kW vs kWh: the simple explanation
Think of driving a car:
- kW is like your speed (60 miles per hour): how much is happening right now.
- kWh is like the distance you’ve traveled (120 miles): how much has happened over a period of time.
Or think of a tap filling a bucket: the flow rate is the kW, and the water in the bucket is the kWh.
The relationship is simple:
Energy (kWh) = Power (kW) × Time (hours)
Examples of kW and kWh
- A 1 kW electric heater running for 1 hour uses 1 kWh.
- A 100 W (0.1 kW) old-style bulb left on for 10 hours also uses 1 kWh.
- A 2 kW kettle boiling for 3 minutes (0.05 hours) uses 0.1 kWh.
| Appliance (typical) | Power | Example use | Energy used |
|---|---|---|---|
| LED bulb | 0.01 kW (10 W) | 5 hours | 0.05 kWh |
| Laptop | 0.05 kW (50 W) | 8 hours | 0.4 kWh |
| Electric kettle | 2.5 kW | 5 minutes | about 0.2 kWh |
| Electric oven | 2.5 kW | 1 hour | 2.5 kWh |
| Portable electric heater | 1.5 kW | 4 hours | 6 kWh |
| Home EV charger | 7 kW | 5 hours | 35 kWh |
Appliance ratings vary by model, so check the label on yours.
How kW and kWh apply to solar
Solar panel and system size is measured in kW
When an installer says «a 6 kW system,» they’re describing its peak power capacity: how much it can generate under ideal conditions. Sometimes you’ll see kWp (kilowatt-peak), which means the same thing for panels. A system’s size is the sum of its panel ratings, so 15 panels of 400 W make a 6 kW system (15 × 400 W = 6,000 W).
Solar production is measured in kWh
What matters for your bills is how much energy your system produces over time, so production is reported in kWh per day, month, or year. A 6 kW system doesn’t produce 6 kWh every hour; output rises and falls with sunlight, and over a year it might produce roughly 6,000–10,000 kWh depending on your location (see How Many Solar Panels Do I Need?).
Batteries have both numbers
A battery has a capacity in kWh (how much energy it can store) and a power rating in kW (how fast it can deliver it). For example, a battery with 10 kWh of capacity and 5 kW of output could power a 5 kW load for about 2 hours, or a 1 kW load for about 10 hours, in theory. In practice, usable capacity is a bit lower than the total and efficiency losses apply. Read Do You Need a Solar Battery? to see how these numbers translate into real-life backup.
Inverters are rated in kW
The inverter also has a power rating in kW (AC). It’s common for the panel capacity to be slightly larger than the inverter rating, which is normal design practice.
How to read your electricity bill
Look for these items on a recent bill:
- kWh used: your consumption for the billing period. In the UK this is often listed as «units.»
- Price per kWh: the rate you pay for each kWh (it may vary by time of day or by usage tier).
- Fixed charges: daily standing charge or monthly service fee, which solar does not remove.
- Demand charges (kW): a few residential plans charge for your highest power draw. These are more common for businesses, but check your plan.
Then find your annual kWh. Many utilities show a 12-month history on the bill or in your online account. If not, add up the last 12 months.
How to size a solar system from your energy bill
- Get your annual kWh from the last 12 months.
- Find your local solar yield in kWh per kW per year. Typical ranges are about 850–1,000 in the UK and 1,100–1,800 across the US, depending on region (details in our panel-count guide).
- Divide: System size (kW) = Annual kWh ÷ Yield.
- Convert to panels: Panels = System size (W) ÷ Panel wattage.
Worked example
Your bills show an average of 900 kWh per month.
- Annual use: 900 × 12 = 10,800 kWh.
- Local yield: 1,400 kWh per kW per year.
- System size: 10,800 ÷ 1,400 = about 7.7 kW.
- With 400 W panels: 7,700 ÷ 400 = about 19–20 panels.
To size the system for less than 100% of your use (for example, 80%), multiply your annual kWh by 0.8 first. Whether 100% is the right target depends on your export rules: see Net Metering vs Export Tariffs.
Turning kWh into savings
Estimated annual savings are roughly the kWh you use from your own solar multiplied by your price per kWh, plus any export earnings. For example, 8,000 kWh of solar power used at home valued at $0.15 per kWh is about $1,200 per year. Use your own rates, and remember that fixed charges remain.
Common mistakes with kW and kWh
- Confusing the two. A «10 kW battery» is ambiguous: ask for both kWh capacity and kW power.
- Assuming a 6 kW system produces 6 kWh per hour. Real output depends on the sun, the season, and shading.
- Sizing from a single month. Summer or winter bills can mislead you; use 12 months.
- Ignoring future use. An EV, heat pump, or electric cooking can change your annual kWh significantly.
- Comparing quotes by kW alone. Compare the expected annual kWh production and the price per kW installed. See how to compare solar quotes and spot red flags.
Next steps
- Collect your last 12 months of electricity bills and total your kWh.
- Use the formula above to estimate your system size.
- Check how a home solar system works if you want to understand each component.
- Try our solar savings calculator (choose your country).
Frequently asked questions
What is the difference between kW and kWh?
kW measures power, the rate at which electricity is used or produced at a moment in time. kWh measures energy, the amount used or produced over time. One kWh equals using 1 kW for one hour.
Is a solar system rated in kW or kWh?
A solar system is rated in kW (its peak power capacity). Its production is measured in kWh (energy generated over a period).
How many kWh does a 5 kW solar system produce per day?
It varies by location and season. Using a yield of about 1,200–1,600 kWh per kW per year, a 5 kW system generates roughly 6,000–8,000 kWh per year, or about 16–22 kWh per day on average.
How many kWh does an average home use?
It depends on the country and household. As a rough guide, an average US home uses around 10,000–11,000 kWh per year, while an average UK home uses roughly 2,700 kWh of electricity per year. Check your own bills for accuracy.
What size solar battery do I need in kWh?
It depends on what you want to power and for how long. Add up the kWh your essential appliances use over the outage period, or your evening usage if the goal is bill savings, then choose a battery with enough usable capacity and enough kW to run your loads at once.
Why does my bill show kWh and not kW?
Because utilities charge for the total energy you consume, not the instant power. A few plans also charge for peak demand in kW, but most residential customers are billed by kWh.